Communication system employing



NOV. 11, 1952 J R RCE Re. COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION Original Filed May 10, 1945 v V 8 Sheets-Sheet 1 BALANCED M uoauuron I l LOCAL OSCILLATOR 4rrE/vu4 TOR C2 0 III 5 i :5 l l HOMODJNE 77 DETECTOR I! ,/*3 i 1 POLAR/TX" a AMPLITUDE osrggron l J!- l I P l I I PULSE rmgmrrm UL OR GENERATOR c0050 our/w: n

LOCAL 7 2 OJLMXLATOR SIGNAL l} C F/ G. 6 C3 1 ATTENUA TO)? I m F/G F/GQ F/G. /O

HOMODYNE DETECTOR z 7 PHAJE k :H/Frm I 1: we. FIG/2 FIG/3 655%25501? LOW ms:

Jzzzz FILTER ouggur N L M TO M N V E N TOR J. R. P/ERCE MMQZM.

ATTORNEY NOV. 11, 1952 R PlERCE Re. 23,579

COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATIL QN Original Filed May 10, 1945 B Sheeis-Sheet 2 F IG. 3 30/ 1 1 I! V" l W I 3 JHJTIW ii W U Mummy 3;

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. INVVENTOR B J. R. PIERCE WJAM 31 A 7' TORNE Y J. R. PIERCE Nov. 1 1, 1952 COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION 8 Sheets-Sheet 3 Original Filed May 10, 1945 MICROPHONE LOCAL 05C M/VENTOR B J. R. P/ERCE A TTORNEY Nov. 11, 1952 J. R. PIERCE 23579 COMMUNICATION SYSTEM EMPLOYING PULSE cons MODULATION Oririnal Filed May 10. 1945 8 Sheets-Sheet 4 IN l E N TOR By J R PIERCE ATTORNEY J. R. PIERCE Nov. 11, 1952 COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION 8 Sheets-Sheet 5 Original Filed May 10, 1945 I I I P- INVENTOR JRP/ERCE ATTORNEY NOV. 1 1, 1952 J R. plERCE Re. 23,579

COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION I Original Filed May 10, 1945 8 Sheets-Sheet 6 FIG.

RAD/0 RECEIVER AAA LOCAL use/gran IA/VENTOR J R. PIERCE ATTORNEY Nov. 11, 1952 J. R. PIERCE 23579 COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION Original Filed May 10, 1945 8 Sheets-Sheet '7 A FIG. /2

TERM.

LRE R INVENTOR .J. R. PIERCE AT TOEWEV Nov. 11, 1952 J. R. PIERCE 23,579

COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION Original Filed May 10, 1945 a Sheets-Sheet s //V 5 N 7.0,? .1 R PIERCE A TTOR/Vt' 2 Reissuecl Nov. 11, 1952 C OIVIMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION John R. Pierce, Berkeley Heights, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Original No. 2,538,266, dated January 16, 1951, Serial No. 592,961, May 10, 1945. Application for reissue April 12, 1951, Serial No. 220,558

Matter enclosed in heavy brackets I: appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

30 Claims.

This invention relates to communication systems for the transmission of complex wave forms of the type encountered in speech, music, sound, mechanical vibrations and picture transmission [bymeans] by means of code groups of a uniform number of signal impulses of a plurality of different types or signaling conditions transmitted at high speed.

The object of the present invention is to provide a communication system capable of transmitting and reproducing with high fidelity a complex wave form over an electrical transmission path in such a manner that the signal-to-noise ratio of the received signal is materially improved, the frequency band width required for the transmission of the signals being held at the same time to a minimum.

Another object of this invention is to provide improved and simplified methods and apparatus capable of transmitting and receiving signal impulses over a noisy channel and deriving therefrom signals having a high signal-to-noise ratio.

More specifically, it is an object of the present invention to provide methods of and circuits and apparatus for transmitting in succession a group of pulses in sequence representative of the amplitude of a complex wave at successive instants of time.

It is a further object of the present invention to provide improved apparatus for determining the code to be transmitted to represent each of a large number of different amplitudes without the use of complicated counting circuits, arrangements and equipment.

Still another object of the present invention is to transform a series of pulses representing the amplitude of a complex wave at a given instant of time into a single pulse having an amplitude which is a function of the amplitude of the original complex wave at the given instant.

Another object of this invention is to recombine a succession of such single pulses of varying amplitude in a manner to reconstruct a wave form of substantially the same shape as the wave form to be transmitted.

A feature of the invention relates to a sampling apparatus for sampling a complex wave at frequent intervals of time.

Another feature of the present invention relates to methods and apparatus for determining the magnitude of an electrical quantity and transmitting a series of pulses representative of said magnitude.

Another feature of the invention relates to methods and apparatus for building up an elec- 2 trical quantity which is simply related to and a measure of the amplitude of a, sample of a complex wave at a given instant. This quantity may take on the nature of an attenuation, conveniently expressed in decibels.

Still another feature of this invention relates to methods and means for building up [of] this electrical quantity step by step by a multiplicative or a divisive process i. e., on a non-linear basis to a total which is a function of to the amplitude of a complex wave.

Another feature of the invention relates to the use of attenuation to be so built up by an additive or subtractive process on a decibel basis to the total required thus obtaining the equivalent of a compression of the signal.

Another feature relates to the transmission of information, in the form of a code, regarding each attenuation introduced or withdrawn.

Still another feature relates to methods and means for receiving such transmitted information, decoding and translating it to finally yield a wave form which reproduces with high fidelity the original complex wave.

Other features of the invention relate to synchronizing and coordinating the various circuits and equipment at the transmitting terminal with each other and with the circuits and equipment of the receiving end so as to secure proper operation of the entire system.

Briefly, in accordance with the present invention, equipment is provided for generating a control pulse or a group of pulses of predetermined time relation one with another. These control pulses are employed to control a code element timing circuit which circuit in turn generates a series of very short pulses some of which are posifife tand some negative and some a combination of e Wo.

Apparatus is also provided for sampling or deriving an electrical quantity which is a function of the amplitude of a complex wave to be transmitted, this sampling means being under control of the control pulse generator. For each of the control pulses a code element timing circuit generates a series of code element timing pulses and these in combination with the sampling means derive an electrical quantity having a magnitude related to the magnitude of the complex Wave at the time of the control pulse.

This electrical quantity, as illustrated herein, takes on the character of an attenuation which may be varied step by step, operating on the com:- plex wave sample and being controlled by a residual from the sample after attenuation to (16-.-

omitted from the circuit in such combinations as to yield an attenuation related tothe magnitude of the complex Wave at the time of the control pulse. The amount of attenuation is tested from instant to instant as a smallerand "smaller change is made in its value 'to see whether; with-q.

in a certain reference limit and at each step, it is in excess of or is below the desired magnitude. If, for example, the attenuationaftenany change is such that when operating inconnection with the complex wave sample the attenuation is' in excess of the desired value, the last previous change is removedbbut" if the 'total attenuation is below the desired'value} the" last change is left in and the next smaller change is introduced for trial. In the specific illustration of my invention givenherewiththis'plan 'is followed and if the magnitudeof the attenuation due to the'last addition' leaves a residual ofthe complex wave'less than an arbitrary reference' l'evel; then" the"last added element of attenuation'is removed'and'an on signal is transmitted. 'If the addition still leaves a residual above 'the reference level, then it remains in and info'rmationto-this efiectis passed to" the remote point by the absence of a transmitted signal, i. er, an' pfi signal; the identification of the information'being specified by the time at which the informational pulse is transmitted or nottransmitted.

This'f'orm of comparing the sample with the attenuation is then'carri'ed on step by step [to] as far [a point] as 'may be desired and'in' any case to 'an-extent so that" the granularity of'the signal finally-reproduced at 1 a receiving point" will be withinthe limits of fidelity contemplated for the :system.

While thus setting up a oode'of onandoff" pulses completely characterizing the amplitude of each 'sample of thec'omplex wave, this code of pulses is transmitted.Since'the amplitude of the sample" is-' completely'defined 'by the code combination 'of pulses the receiving equipment isrequiredonly to' determine, in the first instance, the character of each pulse i. e.; whether it is an'on"'or*an'-off pulse. Consequently as long as the received pulse's are sufficiently'above the noiselevel to permit their'character to be r and a'system of attenuations to be introduced ornot introducedpall analogous 'to'those of the transmitting station, are provided. In addition,

decoding apparatus is provided'in which the received pulses are employed to produce an electrical quantity having a polarity and magnitude similar to-those of the complex' wave sample of the transmission 'end of the system and thus the "complex wave is' reconstructed from a succession of such reproduced wave-samples.

The invention itself both as to its organization and method of operation together with other objects-and features thereof will be better understood from the following description taken with the accompanying drawings in which:

Figs. 1 and 2 show in [function] functional block form the various elements and the manner in which they cooperate to form an exemplary communication system embodying the present invention. Figs. 3 and 4 illustrate the timing and nature of the pulses and waves characteristic of myisystem. Fig. 5 is explanatory of the method the amplitude of a complex wave sample.

8 to 13,-when positioned as shown in Figs. 6 and and apparatus used for measuring or determining Figs.

7, give in detail the various circuits and equip- "I'nentof an exemplary system embodying the pres- '---ent invention. Fig. 14 relates to a modification of a portion 'of Figs. 9 and 11.

Referring more specifically to Fig. 1 let M be a modulating function representative of any complex wave-such as a" speech,= tel eg' rapli or picture wave; a small' 'portion"of*whichis"indicatedlby curve 3! of Fig. 3. A coder or modulator sends out a series of signals after each of the times tm, tm+-1=tm+'r; tm+2-=tm+2T and so forth-*Each series following a pulse-which samplesthe wave, consists of 1+n signals of the onofi" variety. (The signals need notbeon or'ofi but each-signal distinguishes between two positions calledflon and off. These might be positions of time, frequency or amplitude. On might be transmitted as a current being off and off? asthat of current being on.) The first signal of the series or group'is used to tell the polarity'ofthe function M atthe time the sampleWas-taken. The next n signals of thegroup specify the magnitude of- M-at .the time-the-sample-was taken with respect to'some arbitrary reference'levelof 5 voltage or current. These-n signals may be-of such a character and 1 so combined iadvantageously on a binary system and so -described in this specification for illustrative purposes -as to represent a large number of'different amplitudes. Thus, for thecase of eight signalsthere maybe sent; between'tm and tm+1, the series of signalson; on; on, off, on,'off, on, off,-or 000-10101. The first zero'carries'the information that 'M- isof negative polarity andthe remaining signals-that M- is of amplitude 10.5-"decibelsabove reference level, this value-being obtained by binary-counting. -Here the smallest element of change has been taken as one-half decibel.

One specific means for'obtaining such aseries of signals isillustrated in the block diagram -of Fig, l. The over-all functioning" will be described first'and the possible nature ofthe block units will be described later.

While the functions described could-be-carried outwithout frequency conversion,--grounding problems are -simplifiedby first impressing the modulating function M on a balanced modulator I of Fig. 1, together with the output of alocal oscillator II of frequency in, high compared with the frequencies occurring in M. At the time its shown on Fig. 3, a pulse-which will be-referred to as an M pulse, from a pulsegenerator VI is impressed on modulator I.- During the-interval T between tmiLIld tm+1-, when-another 'M pulse is impressed on modulator L- the outputcurrent- Im has a frequency in and an amplitude proportional to M at-tni, beingpluswhenlvl is plusat-tm and minus when M is minus at tel. At tm+ranother sample of the complex'wave ista'ken ana ror the next interval T" the-modulator output has the same frequency f0 and the new amplitudej This is indicated in Fig. 3 in which the second line 302 shows the potential across a condenser (later to be described) and the third line 303 shows the oscillations of frequency fa associated with the successive M pulses. It should be noted that if M is negative, as in the last group shown in line I, this is recorded by a reversal of phase of the modulated current of frequency to.

I111 is fed to a homodyne detector IV associated with attenuating devices A1 An. Local oscillator power from II is also supplied to IV. At the output of IV there is obtained a voltage or cur rent which increases when M increases, is minus when M is minus and is plus when M is plus, although this output need not be linearly proportional to M.

The output of IV goes to polarity and amplitude detector V. This serves two purposes. First, after a pulse is applied to modulator I at tm and after a resetting pulse, later to be described, puts the attenuation of attenuator III at a minimum, a-pulsels applied to V. If the polarity of M at tm is negative an impulse is sent to the transmitter VII and this, preferably in conjunction with a pulse from VI, causes the transmitter to send an on" signal. If the polarity of M at he is positive V sends no impulse to transmitter VII which in turn emits no signal, meaning ofi.

Second, the detector V serves to measure the out- 1 put of the homodyne detector IV and to control the operation of the attenuator III in response thereto, as will be later described in detail.

The attenuation of the attenuator is controlled by a number of resistances R1, R2 Rn. have on and oil conditions corresponding to the digits of the binary numbers specifying the level of M at tm or other sampling time with respect to the arbitrary level. Thus, in the example previously given of 1+n:8 the amplitude is specified by 7 digits. If one of them is beyond the binary decimal point then turning the various attenuations A OE and on in this case changes the attenuation by the amounts listed below:

A1, 32 db. As, 2 db. A2, 16 db. Ac, 1 db. A3, 8 db. A7, .5 db. A4, 4 db.

These seven attenuations are thus capable of producing attenuations covering a range of 63.5 decibels in one-half decibel steps. Addition of one such channel roughly doubles the decibel range of attenuation available.

The attenuators A1 trically by control circuits C1 Cn. These control circuits derive input from (a) the pulse generator VI and (b) the amplitude output of polarity and amplitude detector V.

The character and timing of these pulses are shown in Fig. 4 which is an expansion of the time interval between two sampling pulses or the time required for one cycle of operation. Here the second time interval T is shown. The negative portion of the M pulse resets the modulator I and the positive portion immediately thereafter takes the sample of the complex wave. At the same time the initial negative pulse shown on each of the curves 1n of Fig. 4 places all As inthe minimum attenuation position. In the meantime the sample of the complex wave operates on the homodyne detector IV.

For instance, at tm+1 a setting pulse place all As in the minimum attenuation position. Then a polarity pulse P is applied to V and the polarity of M at tm+1 is transmitted. Following this a positive pulse is applied to C1. This changes A1 and puts in say 32 decibels of attenuation. Immediately thereafter a negative pulse is applied These An are controlled electo C1. If the amplitude of the output of IV is still above the arbitrary level upon the insertion of attenuation the position of A1 does not change and the attenuation stays in. No pulse is sent to the transmitter VII and no signal is sent, indicating off. If, however, the amplitude falls below the arbitrary level on the insertion of this attenuation, the position of A1 is changed back by the negative pulse and at the same time a pulse is sent to the transmitter VII. This, perhaps in combination with a pulse from pulser VI, causes VII to send an on signal.

Next, positive and negative pulses are applied in turn to C2 C11 resulting in either the insertion of the steps of attenuation they control and transmission of off signals or in the insertion and subsequent removal of the respective attenuations and transmission of on signals.

After the l+n signals specifying the polarity and amplitude at tm+1 have been sent, VI sends a resetting pulse to the controls C1 C11. This puts all As at the minimum attenuation position. This pulse sends a marker signal T from the transmitter denoting the end of one interval and the beginning of the next. This pulse is simultaneous with the negative pulse applied to modulator I.

In a review of the system it will be convenient to refer in further detail to Figs. 3 and 4 showing the pulses required from the pulse generator VI. At the top there is shown a series of pulses coming over the channel M to the modulator I and a sample 3&2 of the complex signal wave 39! at the instant of each modulator pulse is taken. Operating on electrical elements the sample causes the modulator to emit, until pulsed again, at current IM or" frequency in and amplitude proportional to M at the sampling time. All this is indicated in the second and third lines of Fig.3. A polarity pulse supplied to polarity andamplitude detector V arrives an interval later and this is succeeded by pulses over channels I, 2 n which operate on the attenuators A1 An. There may also be a pulse channel T to the transmitter as shown in Figs. 8, 9 and 10, necessary if the o signals are transmissions and not merely omissions. All this is shown in Fig. 4 which is an expansion on a time basis of one of the sampling periods of the system.

At the time of the M pulse, channels I to n are pulsed with strong negative pulses setting A1 A11 in the minimum attenuation positions. Next the polarity and amplitude detector V is pulsed by the polarity pulse P. This results in pulse to transmitter VII if the polarity is negative and an on signal from the transmitter, or no pulse to VII if the polarity is positive and an 01? signal from the transmitter.

The n amplitude channels are pulsed in sequence; first positive and then negative. positive pulse inserts attenuation, the negative pulse removes attenuation if the attenuated output of IV has fallen below the arbitrary level and in doing so sends an on signal, otherwise the attenuation stays in and there is an off signal. As shown in Figs. 8, 9 and 10, the channel T applies pulses to transmitter VII at all times whether an on, off or a marker signal is to be used. These pulses will give an on signal;

from the transmitter in combination with a pulse from A1. An or V.

The

by'ispecificf illustrationstof circuitstol accomplishs the: desired. ends, butiitis-tox be understood :that. they are illustra.--

These-will new be'describedi tiveonly andzthat my. invention isnot limited to a the specific circuiti arrangement shown. 1

Flora a; description of; the transmitting end of mynsystcmreference. should he made to FPigs. 8,9. l and 10in which thevarioustblockcomponents-of.

Fig-.41 are:represented by'thesamezRoma-n numebers.

The-icomplex wavee to be transmitted iszime;

pressed on the balancedmodulator systemsI, this complex wave having come from any suitable sourcetsuch asa microphone 8il5tl1roughr appropriate zterminalt equipmentafiim. .Itis then sam-n pled periodically and goes .;through th cprocessztou be-desc-ribe'd in further detail.

Puise generator It: will; be advantageous to nowdescribe;- .the pulseigenerating systemNl; one form of which is shown: in: detail. in. Fig. ID...- The, first; controlling elementgi-nithis .portion or the systemic a relaxae tion: oscillator comprising agasytube IBi-El. This.

relaxation; oscillatori is of a form well known. in; the: :art and includes a resistance; I-t I I for charging a condenser IliIZ from the battery ItI3. .'As,-. sunning: that, to start with, the condenser 1M2 is discharged then on closure of the. circuit it is charged. at a rate determined by the. resistance Illl l. ,Whenthepotential of the condenser and consequently of the plate of tube; IEli-fl rises to a firing value, the condenser suddenly discharges through the; tube. and resistor WM; The durae tion. of the discharge is short and gives rise. to'a sharpgpositivepulse across the resistor I cm; The;

duration of this pulse and the rate at which itis followed by identical pulsescan be completely controlled by the parameters of theucircuit; in

particular, by thevalues of the elements IdII to HEM takenwith the potential orthe grid .oftube Hill) as, determined by the potentiometer-1M5.

While any of several forms of relaxation circuits:

may be: used at this point the one shown is simple and satisfactory. Its operation is more fully describedin many places, suchas on page 1840f Ultra-High Frequency Technique, by Brainerd et al., published'by Van Nostrand Company, 1942;

The positive pulse from IBM is nowused to control theemissionof pulses-to variousparts of the circuit. This is accomplished by connecting across; the element lflld a delay circuit IBIS made up of. identical sections of inductance and ca.-

pacitanc-e connected seriatim. Av positive pulse travels through this network, this-time of arrival atceachsection being uniformly spaced and giving rise'to corresponding [positive and negative] pulses going out over channels I, 2 purposes to be described. The delaynetwork'is terminatedby a load I946 of proper value to suppress any reflected wave.

The parameters of the relaxation oscillator maybe adjusted so thatzpulses are derived across resistance IBM at any frequency desired. For the purposes of myinvention it is preferred to hav a sampling frequency higher than that of n for.

th highest frequencycomponent. in the complex wave to be transmitted. If, for example; this wave is to be a speech wave and it is desired to transmit all components up to-4,000 cycles then there should be at least two samples per cycle of this highest frequency component. A suitable value,- therefore; for the I relaxation oscillator frequency would be 8.000. cycles although: a

her .valuemaybeusedif desired.

By. ,me3;!lS lOf theldel-ay circuit: .or ftimestick 'ii I 0. I562, one has-: available. atth'e ends -ofv therespec-lz. tivea sections, positivesrpulses similar: toi-thatr-ini-- tiated in IBM and spaced in timer one after the vothenby an interval determined byrthe elements in a section of the. timesti'ck. At. timetmiwhen the-pulse is formed at Ifll'4itis transferred ima: mediately-to-tube tMOaand is then transferred to mod'u'lator I asithe M pulse of Fig; 3. The'functionr-of 'this pulse. will bedescribedhereinaften.

Atv-onerelementz of time later the positive pulse. from HIM will have reached the point- P onthew timestickand.thiswi'll be identified as the? pulse.

This .P' pulse operates on -the grid of tube 1-021 to give. a positive. pulse. over' theresistor 1022 which pulse is transmitted to the. polarity and amplitude detector V, appearing there i as a posi tive pulse of a form and at: a time indicated by line P of Fig. 4.

At the end" of succeeding elements of time the positive pulse .from IBM- will arrive at= points n of the timestick andthen are trans-- 1 I, '2- ferredrespectivelyto-the gridsof'tubes I081} I982,

mi-etc; Eachof the'se pulses in its subsequent path is converted into a positive-negative'pulse ina manner-and for the purpose hereinafter tiescribed;

In add i tion to-these pulses it is desired to=-send-- a groupof corresponding pulses tothetransmit ter' VII'which pulsesiwill be identified as 'I'pulses" and areindicated'inthe bottom line oi -Fig. 4'. -It i1 will be noted that the first-pulse inthis cycle t longer and of greater amplitude'tha'nthe -subse-- quent pulses, this-for reasons-which will appear later.- The first Tpu-lse may, for: instance'be approximately twice the length of the subsequent pulses. For'the formation-oftheT pulseszachain of tubes "324- to: I029 is provideda The grid of tube I02 is operated. on directly by" the' pulse In the cathode circuit there is in-* from M. cluded the resistance Ii3l paralleled bythe condenser I032; A positive pulse is generated across I83! and the duration of this pulsepwould ordinarily be the same as the durationrofthe M pulse. However, the addition of a condenser I832 will lengthen this pulse and the condenseni's so chosen. as: to approximately double the duration.

The positive pulse from- -I'II31.is applied-to-th'e'grid of. .I 025;. the cathode circuit ofwhich includes the resistor I030. There is then transm-itted overthe- T channel to the transmitterVIL'at the time-of the M pulse on the timestick a positive pulse-of approximately twice the duration of the M pu'lse. The'relative magnitude of this pulse can be controlled by adjusting the resistance HH-l'in series with. the timestick H3165 As thepo'siti've pulse over-10M reachessuccessively the points P n the grids of-the tubes IOZB't'oIflZB-areope-rated: upon and setup positive pulses across ID30 which is a common cathode resistor for all the tubes 1025 to "329'. Consequently there-is transmitted the desired series of positive pulses tothe transmitter'VIIindicated inFig. 4'. For thepurpose of synchronizing the'sepul'ss with 'theremainder of the equipment, delay circuits may--- beiintroduced'wherever necessary: On'e'such'de laycircuit isshown inthe'T'channel' at H159.

Following the M pulse and the-P pulse the number of sections in the timestick will'usually be made equal to the number of digits required for setting up-theamplitude code to be transmitted.

from VII; If'there should'be n of these then the' number of-possible codes by permutation'of"on" I Thus, if the*'-num- 7 ber ofrdigrts inthe code is seven this wi1 1 make and oif signals would be 2 1' possible 128 combinations, so that in the system it will be possible to discriminate between amplitudes of 128 different values.

Associated with conductor I for the timestick and tube I08l is a circuit comprising tube I06I and transformer I01I. The tube I'6I is shown as a double triode. The grid of the left-hand section of this tube receives a relatively large positive pulse at tm which is then converted at the plate to a negative pulse. This negative pulse is transferred through transformer l01l as a negative pulse to the control of the corresponding attenuation [device] device AI in Fig. 9 and serves as hereinafter described to set this attenuation element to a minimum attenuation. Shortly thereafter a positive pulse arrivin over conduc tor I to tube IIlBI is inverted and appears as a negative pulse on the grid of the right-hand section of tube I OBI. However, the load circuit of tube I 08I includes the inductance I09I which causes the negative pulse generated on the plate of will to be immediately followed by a positive pulse so that the pulse arriving on the grid of the right-hand section of I06I is a negative-positive pulse. This pulse in turn is inverted by the right-hand section of tube IEESI to a positivenegative pulse which is then transmitted through the transformer Hill to the control circuit of attenuator Al. The character and timing of this positive-negative pulse is that indicated in line I of Fig. 4.

A similar circuit is associated with each of the conductors 2, 3 n to give at the time of the M pulse a relatively large negative pulse to the corresponding attenuation control devices setting each attenuator to a minimum attenuation and at a later time transmitting a positive-negative pulse to the control devices of the respective attenuators in the same manner and at times indicated by lines 2-n of Fig. 4.

Various electrical elements appear in the circuit associated with each of the tubes thus far described. Thus across the primary of the transformers I01I, etc., there appear the resistor r1, re, and r3. These are present elsewhere also and they are for the purpose and of a value to sharpen the pulses which are transmitted by the corresponding transformers and to suppress their differentiating action, thus preventing the setting up of an additional reverse pulse. Other elements all serve purposes well known to those skilled in the art and need not be described further.

Modulator I A description will now be given of the modulator circuit I as shown in Fig. 8. A com lex wave made up of numerous freouency components, the hi hest one of interest being for the present taken as 4,000 cycles, arrives at the primary of transformer 8I0. At time tm a ositive pulse arriving on the grid of tube I020 is inverted by transformer IUIB and is impressed as a negativepositive pulse on the plate and cathode of diodes 8H and till. The negative pulse causes current to flow through BIZ and discharges condenser 0. When the positive pulse arrives immediately thereafter it causes current to flow through 0H charging the condenser M4 to a definite potential, this potential being equal to this positive pulse plus that of the amplitude of the complex wave M at tm minus a constant bias potential determined by battery 8IB. The grid of triode 820 is held at this potential minus a bias potential due to 82! until another pulse is applied at tm+1. The output of the amplifier tube 820 appears as a voltage across the resistance 825 and controls the operation of a conventional balanced modu later of any suitable form. As here shown it involves two varistors v1 and Vz. Associated with this balanced modulator is also a source of local oscillations II of frequency f0 high compared to the pulse frequencies present in the system. In a manner well understood inthe artthere will appear in the secondary of transformer 830 'a wave of carrier frequency fo, the amplitude of which will be proportional to the potential across resistor 825. Thephase of the voltage in the secondary of 830 reverses as the voltage across 825 reverses. The output of the secondary of 830 is connected to the grid circuit of a pentode 835 which, in turn, yields an output current Img 'l his current is almost independent of the load consisting of a resistor 900 and an antiresonant circuit 831 [is] in parallel and associated elements to be described hereinafter. It also gives a proportional voltage Cm across the resistor 900.

Local oscillator II The local oscillator II may be any one of the suitable forms well known in the art yielding a substantially sinusoidal output of reasonably constant frequency.

The load of the pentode 835 includes the plurality of attenuators connected in tandem and shown in Fig. 9. These attenuators comprise a plurality of tubes 945, 965, etc. connected in tandem the gain or loss being determined by the amount of controlled attenuation introduced. The purpose of these attenuators is to receive at the input circuit of the first attenuator a voltage proportional to the current Im and to reduce this voltage step by step to, as nearly as possible, a certain reference level, all in the manner hereinafter described.

Homodyne detector IV Associated with the last stage of the attenuators is the homodyne detector circuit IV, the purpose of which is to demodulate the arriving'c'urrent of frequency f0. The homodyne detector comprises a tube shown as a triode L the purpose of which is to serve as an amplifier but'stiil more to be of a character to add no appreciable attenuation to that which has been introduced in the load circuit of the last attenuating pentode 9B5. Associated'with the output of SDI through the transformer 903 is a demodulator circuit comprising two varistors 904 and 905. It is supplied with oscillations from local oscillator II. The output of this modulator will give a voltage across resistor 901 of magnitude dependent on the amplitude of Im as modified by the attenuators. As long as the high frequency current Im is of one phase as represented in the first three sections of the third line of Fig. 3 one terminal (say the right-hand terminal), of resistor 901 will be positive and this will be referred to asa positive out is used for detecting the polarity and amplitude of the complex wave sample.

Polarity and amplitude detector V The polarity and amplitude detector V is shown in the lower box of Fig. 9. If the homor ,ever reslstor fljlcaused by the flow-of current V .sinewd t ct o t t o r. the res stor 3M and 908. (is above: some arbitrary reference level con- .t t ll b e,--by bias 9. currentflows in one-. 1- the othepofzthe triodes 524. or 1925. If the hornsdyne-putput is in-the direction to ;raise .'the potenfina io the-grid has and has suflicient ampii" ,tude tl -enthe grid attuned, will be highlynega- 'tive. fT herefore ,a .pulse coming :fnom tube H32! and applied in'the' ,gridcircuit through transwiorm r-ml w ll cau en puls in t e. utput f tu .-.& -6,-:and-.n .puls r n the line lead n ther .mm t e t ansmi ter VII. ithe homodyne eou eutjh s the-oppos po ar t th d f 6 md ljlnr b e 01113 slightly negative and the pulse oughetlansfoiinier-ll'l willv cause plate current fl w send ng 53.911159IVERH'ZHISZEDHHBWQZB,

' I; the ut ut f'athe modyne detector is lar er t nlthevarbitrs r-veln in. eitherd t on cur- WH Jfi W iI :.d Q 9 because o the .drop

through therespective tube.9.24 or 825.,thus amplying a positive bias tothe attenuator controls and preventing a negative -pulse coming from the zp s en r t r t a particular; tan rum-from movin the att nuation.

Attenuator 'IIEI ln Fig. 19 there are shown a pluralityiof attenuators and attenuator control-circuitsone. for each edi'git.insthewamplitude :code. Three such units g are s wn: bu ina mu h: as heir action is idenp H xcept-iortiming. it is necessary ie-describe on ome of these,;identifiedby the -block I-II, which includes the f rst attenuator of cries. ,E ich ;10f the .units 1H 2comprises a la tenua ion circuit as air and a lmg circniteas C Each: attenuation circuit ..S D i h t o n circuitgzbetween a air of tubes 945, 955, etc. Thesetubes: areshown as essentially constant current devices and the voltage which is passed from one tube to the next -is..:proportional to :theeimpedance of. the coupling network. By :controlling the resistance :of .the -grid resistor in that networkanydegree of attenua-tionmay be-obtained.

Each of-the:tubesf945,.:965,-.etc. will have a gain which:isdeterminedamong other things by the :impedance ofathe couolingnetwork from one tube -to: the next. 'This-will'ibe a-maximum when no *attenuation isi-ntroduced and theover-all gain for' the series of tubesunder this condition will he -assumed asdeflnite and will take on any predetermined 'valuefrom say 1 to as high a figure as desired. -When reference in this specification -ismade to attenuation; itdsunderstoodthat this trol voltageapplied to terminals a and b cuts off one ormore tubes in the loop or allows them to operate. In this instance the loop is opened by a sufliciently high negative bias on'the grid of itu e 942 Un eruthese condit ns the upl n assis an e o A1 r -tubes 945, .to. 36 isessen- .tially R1+R15 and ,the is aminirnum- If a positive potential of :sufilattenuation, for this stage cient value arrives at, the. point "D then 1i'lheloQP is closed and ifthegain through the circuitconvwining tubBstM .to 9. 53 .ishigh then Rifis-flvirtually short-circuited and the coupling resistance pin; is essentially R1, almost independent of gain. Thus .high. accuracy of control of .Al -.,can. be

obtained independent oftube characteristics.

plural y of. di des. 95 l 9 52,. and 553,.andass0ciated el men s. When a. pc itivepul e .oyerchannel I "supplied throu h tub s. H.161 and. 1 061.101 Eig...1Q,.. .apn ied the .diodefiil conducts, charging .thepondenser fllifliappl sa positive. poten- ..tie.1 .to point blan sin theieedback loops thus increas n the-attenuation A1. Immediate- -ly thereafter ane ative pulseis applied through i ansformerfifi todiode .952. If, this is thennly pulse... present on diode 9 52 ,that is, if insufficient positivebias arrives from the. amplitude andpolaritydetector V, meaning a homodyne. detector output below .a certain reference, the negative pulse. discharges condenser 954 and-opens .the feedback loop thus removing ,the attenuationAi. At the same time current flows in resistances and through diode 953 sending a negativepulse to the transmitter resulting in the transmission of an. ion.signal. Ifthere is apositivebias. of sufiicientmmagnitude from the amplitudedetector through diode 929., meaning alargehomodyne output, the negative. pulse, through transformer S55 is not sufficient ,towcause current to ,flow thrpushdiode 9.52. conseq en y the attenuation remains in amino pulse is sent to .the transmitter.

If the attenuation A1 remains .the, residual signal. arriving ,at tube MI is correspondingly attenuated. With the, arrival of the next pulse, correspondihgitod channel 2, the sample ishagain tested by theeintroduction.,of...attenuation A2, followedby its removal if necessary as determined by the test.

Thisis madeclearer by reference to Fig. 5 in Whichior simplicity it is assumed that, exclusive of the polaritypulse, a 7eplaceamplitude code is to housed on .a binary system. This makes it possible to discriminate between about l28-different amplitudes on a decibel basis. If the amplitude is to be expressed in terms of a reference leveleoanfl ifthes nal-lest.chanseto be observed is one halfdecibel then the tangent: amplitude on a power basis would be from. .5 decibel :to

[63 5] 63.5 decibels, as sug ested on the tableiof page 8. The first attenuation wouldintroducea 3 loss of 32 decibels, the next a loss of -16 decibels and so forth and' the :operationof the circuit is such as to introduce a totalattenuation-in decibels which is-as nearlyas possible equal "torthe .decibe1 amplitude of-em as compared with the reference level 60.

If, for illustration, the sample amplitude em appearing across the resistor 900 is lfiifiidecibels above 80 then-the introduction of A1, onthearvri-valofpulse overchannel l, with an :attenuation of 32 decibels will leave a residual "of 1.4.6

decibels, as indicated at a. This is substantially above reference level and therefore A1 stays in.

Had it been less than zero decibelsthe attenuation would have been removed. The positive .pulse.,.o.ver channel 2 now. introduces Aaof atple.

tenuation value 16 decibels, giving a total attenuation of 48 decibels and a reduction to -1.4 decibels, which is less than reference level as indicated at b and so thenegative pulse removes the attenuation. The pulse over channel 3 now introduces A3, of attenuation value 8 decibels, giving a total of 40 decibels and a reduction of the residual to 6.6 decibels as shown at c, more than reference level and so As stays in.

The next step A4, of attenuation value 4 decibels, gives a total of 44 decibels. The residual is now reduced to 2.6 decibels and therefore A4 stays in. On the next step A5, of 2 decibels value, is added and brings the total attenuation to 46 decibels with a residual of .6 decibel and therefore A5 remains in. As introduces an additional 1 decibel giving a total of 4'7 decibels and a residual of --.4 decibel, less than reference level and therefore As is removed. Av, which uses up the last step in the 'l-cligit code assumed, introduces .5 decibel for a total of 46.5 decibels. This leaves a residual of +1 decibel and so A; remains in. Thus the total attenuation introduced in front of tube 90! is 32+O+8+4+2+0+.5=46.5 decibels and a corresponding code of off, on, off, oif, off, on, off (or the equivalent of will have been set up. The marginal operation of the circuits is so adjusted that since the amplitude is in excess of 46.5 decibels, the negative pulse for A: would not be suflicient to remove it. If slightly less than 46.5 decibels, the negative pulse would have removed it.

It will appear from the above explanation that the final total attenuation introduced is proportional to the amplitude of the sample, within the l smallest adopted decibel step as applied to the reference level unit. The percentage accuracy will be independent of the amplitude of the sam- This may be shown as follows:

Let em=the input voltage to be measured. eu=reference voltage.

If the introduction of 11 decibels of attenuation leaves a residual slightly above 0 decibel level, this corresponds to an input voltage e1, where If the introduction of (n+.5) decibels reduces the residual to slightly below 0 decibel level, this corresponds to an input voltage e2, where Therefore maximum error for Bm= 6 per cent. This is the same for large or small input amplitudes Within range of en to On completion of this process the voltage delivered to the input of tube 901 has been adjusted as nearly as possible to the constant arbitrary reference voltage e0. Since the total attenuation Al: is given by the binary number represented by 11 off or on signals, this number also expresses the amplitude of current Im and hence the amplitude of M at the sampling time.

By using a code of fewer digits the system would be simplified but at the expense of range or granularity. By extending to a code of a higher number of digits any degree of fineness of granularity and corresponding fidelity may be obtained.

It will be observed that this mode of building up attenuation to match a sample amplitude is essentially a multiplicative or divisive process starting with large decibel steps and going to smaller ones until the last degree of fineness contemplated has been attained. Being multiplicative or divisive on an absolute basis it becomes additive or subtractive on a logarithmic, that is a decibel basis. This multiplicative or divisive characteristic is peculiar to this system and constitutes a distinguishing feature, as is'pointed out in the appended claims.

Transmitter unit VII During this procedure there has been arriving at transformer a series of pulses over channel T, one for each pulse from the pulse generator, timed as indicated on the bottom line of Fig. 4. These pulses may be used to operate on a grid of the tube 855. In addition, there [arrives] arrive at the transformer 852 certain pulses, one for each on pulse, relating to polarity or indicating that one of the attenuators has been introduced and then removed. No pulse will come to the transformer 652 if an attenuator has been introduced but not removed, this corresponding to an ofi signal. The secondary of the transformer 852 operates on a second grid of tube 855 and this tube in turn controls the transmission or absence of transmission over a suitable medium to a remote station. In Fig. 8 the tube is shown as controlling a transmitting terminal unit 860 for a radio channel on a suitable carrier but it is to be understood that the pulses coming from the tube 855 may go directly to any suitable transmission path such as a pair of wires, a coaxial cable, etc. In such cases it is not necessary and may not be desirable to use the pulses for modulating a carrier. The connections of the transformers 856 and 852 are such that a pulse arriving at 856 alone will not cause the transmission of a signal but the simultaneous presence of a pulse on 850 and on 852 would be effective in causing such transmission and would correspond to an on signal. This is so except in the case of the first broad pulse of the T series which is purposely made greater in amplitude than the others so that it can operate tube 855 alone. The purpose of the pulses coming over the T channel is to assure proper timing coordination of the trans mission of signal pulses. In some instances such added precaution will not be necessary in which case the T channel may be omitted, including the chain of tubes I624 to H329 of Fig. 10 and the transformer 850. In this case also the adjustment of the transformer 852 alone and tube 855 is such that a pulse on 852 will then be sufficient to cause transmission.

Receiver The problem of recovering the modulation function at the receiver station is somewhat simpler and will be first described by means of the block diagram of Fig. 2. For the example here described pulses forming the signal appear at A for all on signals. A pulse generator VIII is synchronized with the incoming pulses, perhaps paws -byimeans of a marker pulsesuch as the'Mpulse from the transmitting station, so that its operations coordinate properly with the incoming sig- 'nals. This generator sends out pulses to various devices which. also receive the signal. 'ceived' signals alone, or the pulses alone'will not "cause operation. A signal pulse in conjunction These rewith a pulse from VIII will cause a device to 1 operate.

'At, the time when the signal corresponding to polarity appears the pulse generator applies a pulse to a phase shifter IX. If there is anfoif signal; i; e. no signal pulse the phase shifter is set to'shiftphase 180'degrees; if an on signal "the phase :shifteris set to shiftphase zero degrees. The phase shifter remainsin thisposition ;unti1f:receiving:another P pulse.

.The-next-pulse'of thexpulse generator" is sent to the control 0.1 which controls a change of attenuator A1 proportional to that of- A1 of vFig. 1. The attenuation is initially at a minimum. If the received signal is an off signal, attenuation .is'left at a minimum. If an "on signal is received this causes operation and A1 is switched in. The next pulse from VIII goes to C2 at the same time that the signal from A2 of Fig. 1 arrives and so on through An- Thus, the total attenuation is made the complement to that introduced ;in the transmitter of Fig. 1 which gave rise to the an output pulse from the homodyne detector XI nearly proportional to the amplitude M at the sampling time. A reset pulse may follow the pulse to oscillator X. This will reset A1 An and phase shifterIXto the initial position in preparation for succeeding sample codes. The pulses from the detector XI are passed through a lowpass filter XII. If the highest frequency fm in the complex wave is fm= /2T and if the low-pass filter has a cut-off of fm, a signal proportional to M, but delayed perhaps by T seconds, is recovered at the output of the lowpass filter. Here T is the length of a period between tmand tm+1; i. e. between sampling times.

With this brief description of the block diagram of Fig. :2 we may now proceed to a more detailed description-of devices which will accomplish the steps set forth above. It will be apparent to those .skilledlin the artthat there are numerous circuit arrangements for accomplishing this. Certain specificarrangements are here shown .in Figs. 11,

.12 and 13 but these are illustrative for the sake .of concreteness and it is to be understood that .many variations may be made without departing from the spirit of my invention.

Referringmore specifically to Fig. .11, there is shown a receiving unit H06, here indicated as a radio receiver associated with a suitable receiving antenna H05. This unit H06 is a radio receiver of any suitable type including a detector the output of which yields the pulse signals as a: reproduction of the pulses arriving at the transmitting unit VII ofFig. 8. This code pulse message is amplified to any necessary extent as illustrated by the tube i I 08 and the output thereof is shown as going to a plurality of control devices 01 C one associated with .each .01 a plurality of attenuators in a' manner hereinafter to be described.

Receiver pulse generator VIII A derived path from the output of IHJB passes through suitable amplifiers as shown at i HI and I4 and an outward pulse therefrom is used'to control a relaxation oscillator shown in Fig. 13. This relaxation oscillator, centering about the gas tube Hit, may be similar in every respect to the relaxation oscillator at the transmitting station and shown in detail in Fig. 10. Corresponding to each of the units new to 1315 in Fig. 19 there are the units l3! to i315 in Fig.13 Theadjustment of the parametersin the relaxation oscillator of Fig. l3, however, is such that the circuit does not normally oscillate but is triggered by a pulse arriving from tube Hi4. Furthermore, the parameters of this relaxation oscillator are so adjusted that the circuit will be triggered by the first pulse in a group (correspending to theM pulse at the transmitter) after which the oscillator cannot be triggered until'the arrival of the next M pulse.

This is accomplished by the use of the long initial T pulse of Fig. i to which reference has been made. It is to be borne in mind that. .all pulses transmitted by VII are of'the same amplitude. However, since the tube I i i2 is essentially a'constant current device, the voltage built up in the tank circuit Hi3 is proportional to the duration of the incoming pulse and thus the long initial T pulses arriving at I3IE$ and corresponding to the M pulses will be of greater, perhaps double, amplitude and so be able to trigger-the relaxation oscillator, whereas the other pulses in the cycle will not.

In a manner analogous to that of Fig. 10 there is associated with the relaxation oscillator a timestick [3H5 from which a series of pulses may be derived with a time spacing as nearly identical as may be necessary to the time spacing of the pulses derived from the timestick at the transmitting station. This timestick has an additional section giving rise to a pulse indicated by n and delayed only slightly behind the previous pulse. The function of the pulse n will be given hereinafter. The timesticl; is terminated with a suitable impedance I318 to suppressrefiection. Also there is a series oftubes I325 to I329 from the cathodes of which a series of positive pulses is derived [from cathode followers is initiated] corresponding to the pulses from the timestick.

Receiver attenuators XIII The utilization of the various pulses to control the setting up of a series of attenuators A1 An in consonance with those set up at the transmitting station will now be described. For this reference may be made to Fig. 12 and the right-hand portion of Fig. 11. In this latter portion there isshown a series of tubes I M5, H and H35 connected in tandem and analogous to the corresponding circuit'of Fig. 8. The coupling from one tube to the next is a resistance capacitance network, a resistance therein being subject to change whereby the portion of the voltagegenerated inone tube and passed to the next is attenuated by definite amounts, the amount of this attenuation being conrollable in accordance wih the code signal which is being received. Thus, the output circuit oftubellf45 includes the attenuator circuit A1. A1 comprises the resistances R1 and R1 connected in series, the intermediate point being connected to the three stage amplifier comprising the tubes I24I, I242, I243 with resistance capacitance or other suitable coupling, there being a feedback connection from the plate of the last tube I243 through condenser I244 to the grid of I24I. There is either zero or a large gain around the loop depending on whether the control voltage applied to terminals a and b cuts off one or more tubes in the loop or allows them to operate.

One mode of operation will be described in connection with A1. Here, as in Fig. 9, the loop is normally held open by a sufliciently negative.

bias on the grid of tube I242. Under these conditions, the grid resistance in the input of tube II65 is R1+R1'. If a positive pulse of sufficient magnitude arrives at b then the loop is closed and if the gain through the circuit is high, the grid resistance is essentially R1, almost independent of gain, and a corresponding attenuation is introduced. Again, whereas for illustrative purposes the control has been shown on one tube only it may be desirable to control the bias on several or all tubes in order to open the loop completely.

The control portion C1 of the attenuator may take on a large variety of circuit forms so long as it performs the desired function in response to the pulses reaching it and may comprise one or more diodes or combinations of diodes and triodes or multigrid tubes in a variety of ways as will be clear to those skilledin the art. Specifically in Fig. 12 it is shown as a combination of a diode and a triodc. A positive pulse corresponding to the M pulse from the timestick operates through transformer I252 so poled as'to make the cathode of diode I25I negative, whereupon any positive charge on condenser I254 is discharged and the negative bias on tube I242 opens the attenuator loop. This occurs simultaneously on all of the attenuator units at the beginning of a cycle and sets all the attenuators at minimum attenuation.

In due course a pulse from circuit I of the timestick and tube I321 arrives at transformer I256 being so poled as to make the grid of tube I251 positive. However, the plate circuit of I251, which includes the transformer I258, includes a battery I259 with the negative terminal toward the plate so that the positive potential on the grid is not able alone to produce a current through the plate circuit and therefore no charge is placed on condenser I254. This is the condition which exists in case of an off" pulse in which event no corresponding pulse through the primary of transformer I258. It will be recalled that the off signal corresponded to the introduction at the transmitter of attenuator A1 and it is noted that corresponding thereto such attenuation is not introduced at the receiver. In case of an on signal there will be a pulse in the secondary of transformer I258 which is so poled as to give a positive potential to the plate I251. This pulse alone is not suflicient to cause the current to flow through the tube because of bias on the grid. If however, there is received simultaneously a pulse from the pulse generator rendering the grid positive then there will be a flow of current charging the upper plate of condenser I254 positively and so closing the loop of attenuator A1 and introducing the corresponding attenuation. The charge on condenser I254 will persist until the time of the next M pulse [whereon] wherepasses- 18 upon the condenser will be discharged as heretofore noted.

Thus it is seen that attenuator A1 will not be introduced if the corresponding attenuator A1 has been introduced at the transmitting station. If it has not been introduced at the transmitting station, it will be introduced and remain in at the receiving end. Precisely the same operation will take place for each attenuator A1 An after which the combination of attenuators connected in the circuit will be the complement of that at the transmitting end. Thus, whatever attenuation is introduced at the transmitter it will be omitted at the receiver and the reverse.

Local oscillator X The receiving station is provided with a local oscillator X shown in Fig. 11 as H20. This may, but need not be of the same frequency as the local oscillator II at the transmitting station. In other words no synchronism between the two oscillators is required.

Phase shifter IX The phase shifter IX is shown as comprising two triodes I22I and I222 the grid circuits of which are supplied in parallel from the local oscillator H20 through the transformer I223. The output circuit comprises transformer I224 the mid-point of the primary of which is connected to the positive terminal of the battery. The grid circuit of tube I22I contains the condenser I225 and the grid circuit of tube I222 includes the battery I226 which tends to give a positive bias to the grid. Normally, therefore, the transconductance of tube I222 will be higher than that of I22I and there will be an alternating current of local oscillator frequency in the secondary of I224 of one phase. The phase of the current in the secondary [current] circuit may however be reversed by means of the phase shifter.

This phase shiftercomprises two diodes I23I and I232 biased so that normally they are nonconducting. When a pulse from the tube I325, corresponding to the P pulse, arrives at transformer I233 it is so poled as to render diode I23I conducting, givinga positive charge to condenser I225 of such. magnitude as .to give tube I22I a higher transconductance than tube I222, whereupon the current in' the secondary of I224 is reversed in phase. The reversal occurs if the polarity pulse of the code at the transmitter was an off signal, meaning an absence of a received pulse. The condenser I225 is so connected as to retain its charge for the duration of one complete cycle.

If, the polarity of the sample of the complex wave at the transmitter had been negative then an on P polarity pulse will have been transmitted and, in turn, received at the receiving station. A corresponding pulse, therefore, arrive at the transformer I234 in parallel with the transformers I258, etc., at each of the attenuation control circuits. The transformer I234 is so poled that its pulse opposes [that of] the pulse coming on transformer I233 and consequently diode I23I does not conduct, the condenser I225 does not become charged and the phase of the oscillations in secondary of I224 is not reversed. At the end of the cycle, or the beginning of the next cycle, the M pulse from the tube I325 operates through transformer I236 to make diode I232 conducting whereupon the condenser I225 is discharged or reset to normal condition. Through the means thus described it is seen 19 that it is possible to change the phase of the local oscillator current in transformer I224 by 180 degrees.

The output of I224 of local oscillator frequency is impressed on the input circuit I I40 of the first attenuator tube II 45. In the output circuit of the last attenuator tube I I35 there is included the resistance IZIE corresponding to R of Fig. 2. This resistance islarge compared to the resistance Rn of the last attenuator and consequently adds no appreciable attenuation. Thus it is seen that the amplitude of the current in I2I6 and'the potential variations across its terminals, as well as the current in the transformer I2l4, will be proportional to the sum of the attenuations which have not been introduced atthe receiver. Consequently, they will be proportional to the sum of the attenuations which were introduced at the transmitter and therefore proportional to the amplitude of the complex wave sample. Furthermore, [its] their phase will be determined by phase shifter IX to correspond with the polarity of the complex wave sample.

Homodyne detector XI The homodyne detector in includes a balanced demodulator comprising the varistors [2H and I2I2 connected in a standard bridge circuit. This demodulator is supplied directly with local oscillator frequency through transformer I2I3 and also with the same frequency through tube I2 I 5 and transformer I2 I 4. There will then appear over the resistor I2II potential difierences of sampling frequency, each element being. proportional in every respect to the sampling current or voltage at the transmitter. If the polarity of the sampling current at the transmitter reverses, then the voltage across the resistor I2I'I will also reverse.

The last step of interpretation may come through the multigrid tube I2I8. This tube is normally biased to cut-off. However, through the additional section of the timestick leading to the pulse n, previously referred to, a positive pulse is impressed on the triodes I35I and I352 connected in series to give requisite amplification without change of polarity of the pulse. Very shortly after the termination of the amplitude code there arrives then on one grid of tube I2I8 a positive pulse of suflicient value to enable the tube I 2 I 8 for the duration of the pulse n. During this interval then there appears in the output circuit of I2 I8 a pulse determined bythe magnitude and polarity of the voltage over I2 I6, which latter is proportional to the amplitude'and polarity of the complex wave sample.

These pulses will arrive in succession, one for each sample from the complex wave. By means of the low-pass filter I2|9 the undesired high frequency components maybe removed and the resulting wave, passing to terminal equipment and receiver, will be a reproduction of high fidelity of the original complex wave at the transmitting station. I

During the setting up of the attenuations, pulses modulated on a suitable carrier will have been transmitted from VII bearing the information to the remote station on what attenuations are being introduced. The amplitude of each of the pulses so transmitted from VII will be the same and each element of the signal is purely an off and on matter. Since only integers are sent such a signal can be repeated without added distortion or noise [tolin the recovered intelligence even though distortion or noise below a certain threshold level may be present in the repeaters. Thus, even for very high quality transmission the requirements on the repeaters are very low. This makes possible transmission over long paths with frequent repeating. The presence or introduction of noise in the transmission path from VII to the remote receiving station will haveno influence so long as the noise introduced is relatively small compared with the signal being transmitted over the path. Such noise therefore will not appear in the signal later reproduced.

This application is related to my copending application Ser. No. 603,989, filed July 9, 1945, Patent 2,508,622 of May 28, 1950, and differs from it in the manner of building up attenuation. Whereas in that application the conductance steps are additive on a unit or multiunit basis, in this application the changes are on a decibel basis, that is, the attenuations herein are in successive steps of multiples. This has certain advantages one of which is the much Wider range of amplitude which can be covered by a code of a given number of units. Thus in my aboveidentified application for a code of in ,digits the range of selection of amplitude in voltage (or current) is 2 whereas in this system the range of selection of amplitude in decibels in 2. If n=7 .and if the reference level is taken as ea (perhaps in millivolts), then for the copending application the range of amplitude is from 60 to 2 60 (:128 eo, equivalent to 42 decibels). This means .a granularity of en in a maximum contemplated signal of 128 cu; that is, an accuracy within 1 per cent. However, the granularity is still en no matter how small the input, and so for an input as low as 2 co the accuracy is about 50 per cent. In the system of this application the percentage accuracy of reproduction will be the same without regard to the amplitude of the output. Thus, if .thesmallest step is .5 decibel, then the error in determining the value of input voltage will not be greater than 6 per cent and will average about half of this. Furthermore, for a seven-digit system the range in amplitude willbe from as ax 10 (that is, about 1 to 1500). This feature of a nonlinear scale (where the electrical quantity built up is not proportional to the voltage or current amplitude of the sample) is equivalent to a type of volume compression in the coded signal and this is an essential aspect of this invention. The particular non-linearity which has been used for illustration is a decibelscale (the electrical quantity built up is proportional to the logarithm of the voltage or current amplitude of the sample) and this, as pointed out above, not only gives wider range but gives smaller granularity for low amplitude samples.

In considering the above system it will be apparentto those skilled in the art that many variations may be made in'the system thus far described without departing from the spirit of the invention. For example, the attenuation scheme, which constitutes an important element in my invention, may be materially altered as indicated in Fig. 1 1. There the plurality of resistance com- .bina'tions R1 and R1, R2 and R2, etc. is shown as a series of potentiometers connected in tandem. The portion R1, R2 Rn of each potentiometer is capable of virtual short circuit as already indicated. The portion R1, R2, etc., of

the resistances will have to be large compared with the electrically controlled shunt portions if the attenuations caused in the various potentiometers are to be substantially independent. This may be somewhat difiicult to attain and for this reason isolation of the electrically controlled resistances by means of vacuum tubes, as already described in detail, may be desirable.

Other changes may be permissible, some of these having been already mentioned. For ex-' ample, the T channel of pulses may be omitted with corresponding simplifications although with some loss of control. Also, the local oscillator at the receiving station may be on all the time instead of being triggered on occasionally, for even if on, it is ineffective at the terminal apparatus unless and until the tube l2l8 has been enabled by the pulse coming from n supply. Still further, it will be evident that modulation and demodulation features of the transmitter station may be omitted, the sample signals of the tube 820 going directly to the input of tube 835 and directly from tube 9M to the resistor 901, or its equivalent, without the intermediation of the demodulator in the homodyne unit IV. This would mean also the omission of the local oscillator. Corresponding alterations could be made at the receiving station in connection with its local oscillator. In general, however, such omissions or simplifications will lead to sacrifice in operation or quality and it will be a matter of engineering judgement as to how far one may carry out such simplifications.

What is claimed is:

1. The method of transmitting information on the shape of a complex signal wave which comprises taking amplitude samples of the wave at equally spaced intervals, building up an attenuation network for each sample step by step to a magnitude proportional to the sample amplitude and transmitting oil or on pulses in accordance with each step, the steps being related to each other on a non-linear basis.

2. The method of transmitting information on the shape of a complex signal wave which comprises taking amplitude samples of the wave at equally spaced intervals building up an attenuation network for each sample step by step to a magnitude proportional to the sample amplitude and transmitting ofi or on pulses in accordance with each step, the steps being related to each other on a logarithmic basis, the total attenuation on said basis being equal to the sum of the attenuations for the individual steps introduced.

3. The method of claim 1 with the added step of introducing at a receiver an analogous attenuation network, the total steps of attenuation there introduced being the complement of those introduced at the transmitting station.

4. A system for transmitting information on the shape of a signal Wave comprising a circuit for periodically sampling the amplitude of the Wave, means for building up an attenuation network by a series of non-linearly related steps to a value proportional to each sample, means for generating a cycle of pulses for each sampling operation, means responsive to the first pulse of the cycle for controlling the sampling circuit and means responsive to succeeding pulses of the cycle for controlling said means for building up an electrical quantity in coordination with the respective sample.

5. A system for transmitting information on the shape of a signal from a transmitting to a receiving station, the transmitting station comprising means for sampling a signal wave periodically and storing on a condenser a potential proportional to the sample amplitude, an amplifier tube the input of which is connected across the storage condenser, means for connecting in the circuit of said tube a multiple attenuator comprising a plurality of attenuators in tandem to attenuate the efiective output voltage, a pulse generator means controlled to build up the said attenuation step by step by the coordination of the signal amplitude with pulses from the pulse generator until the residual effective output falls to an arbitrary low reference level.

6. A system for transmitting information on the shape of a signal from a transmitting to a receiving station, the transmitting station comprising means for sampling a signal wave periodically and storing on a condenser a potential proportional to the sample amplitude, a constant current source producing a current proportional to the charge on the storage condenser for the duration of the sampling period, atrain of unilateral attenuating devices operating to attenu ate the current, a control circuit for each attenuator, each attenuator adapted to introduce or not introduce its attenuation subject to the control circuits, the control circuits being operated successively by pulses from a pulse generator and introducing attenuation if the residual of the sampl amplitude is in excess of an arbitrary small amount.

7. The combination of claim 6 characterized by the fact that the train of unilateral attenuating devices comprises a series of amplifier tubes connected in tandem, the coupling circuit from the output of one tube to the input of the next including a coupling resistance a portion of which may be substantially short-circuited to introduce a designated amount of attenuation,'the introduction of said attenuation being subject to the control circuits.

8. The combination of claim 6 characterized by the fact that the train of unilateral attenuating devices comprises a series of amplifier tubes connected in tandem, the coupling circuit from the output of one tube to the input of the next including a coupling resistance a portion of which may be substantially short-circuited to introduce a designated amount of attenuation, the introduction of said attenuation being subject to the control circuits in such manner that if the residual of the sample amplitude at any stage is in excess of an arbitrary small amount, the next step of attenuation is introduced and if below said arbitrary amount, it will be first introduced and then removed.

9. A system for transmitting information on the shape of a signal wave comprising a circuit for periodically sampling the amplitude of the wave, means for building up an electrical quantity by a series of steps related on a logarithmic basis to a value proportional to the signal amplitude, means for testing the polarity of the sample, and means for generating for each sampling operation a cycle of pulses, means responsive to the first pulse of said cycle for controlling the sampling circuit, means responsive to the second pulse of said cycle for controlling the means for testing the polarity of the sample, and means responsive to succeeding pulses of said cycle in coordination with the sample for controlling said means for building up an electrical quantity.

10. Apparatus for transmitting a complex wave form comprising means for setting up a series of groups of permutatively coded signaling pulses, the series representing a successionof instantaneous amplitude samples otthewave form. and each pulse of a group representing a d-iiierent fraction of the amplitude of a wave form: sample, the meansthereiorcomprising a 'pl-uralityo'f attenuators adapted to oe-connected in tandem, each element of attenuation being proportional tea-fraction of the corresponding sample amplitude 'a nd'the-sum of theattenuationsintroduced being proportional to the instantaneous amplitude ofthe waveform.

11. Apparatus -for reconstructing a complex wave form from a series- 'of groups of per-mutatively coded signaling pulses, the series represent'ing asuccession of instantaneous amplitude samplesof 'the complex wave form, each. pu lseoi a-group representing a difierent fraction of the amplitude of said wave form sample, the-means therefor-comprising a plurality of attenuators to be introduced in tandem, the total attenuation being proportional to 'thesum of the-individual attenuations, this sum subtracted from a constant being proportion-alto the-instantaneous amplitude' of the complex wave sample.

12. A-system for transmitting iniorma tion on the shapeo'f a signal wave-from a transmitter to a receiver station, the transmitter stationcomprising acircu-itfor periodically sampling the amplitude of "the wave, a 'current 'source adapted to deliver a current proportional to-the sample amplitude and constant for the duration of a samplinginterval a load supplied with current from said "source, a circuit "fox-"transferring the effective voltage'across the load to apolarity and amplitude detecting circuit, a plurality of I n attenuators'to be connected in tand'em in the load circuit of said current generator and thus reducing the vol tagereaching the-polarity and amplitude detector, a control circuit foreach attenuator'; a pulse generator adapted to generate cycles of pulses, one pulse in the cycleservi'ng as a marker pulse and as timing thesam-pling of thesignal wave, another serving as a polarity puls'eto'cooperate in testing the polarity of the sample amplitude, the remaining pulses operating successively through the control circuits of theattenuators in cooperationswith the residual amplitude reaching the amplifier Y detector to introduce one or more ofthe-attenuatorsand thus to reducethe residual reachingthe amplitude detector to an arbitrary small value, a circuit associatedwith each control circuit'to transmit an elf signal if its attenuator-islert in and an on signal if 'it-isremoved, the n attenuators being graded in size'from the-smallest of value-Ao,the largest being first tested for introduction, the system so operating that the total attenuation ona decibel'basi's introduced and left in at the end of the cycle is proportional to the sample amplitude.

-l3.- A system for transmitting information on the shape of a=-signal-wave from a transmitting to a receivingstation, the transmitting station comprising a circuit for periodically sampling the "amplitude of the wave and storing a charge on a condenser proportional to the amplitude, a constant current source controlled iby the condenser charge and adapted to deliver a current proport-imral to the condenser charge andconstant, for! the duration of a. sampling interval, a circuitfor' transferring the voltage acrosslth'e load to a polarity and amplitude detecting-circult, a plurality of n attenuators to "be connected in tandem in the'loadcircuit of thecurrent genorator to-reduce the voltage reaching the polarity and amplitude detector, a control circuit for each attenuator, a pulse generator adapted to generate cycles of pulses each pulse consisting of 2+n equally spaced pulsesthefirst pulse'in the cycle serving as a marker pulse and as timing the sampling of the-signal wave, the second serving as a polarity pulse for timing theoperation of the detector to test the polarity of the condenser charge, the next pulse operating through the control circuit or thefirst and largest attenuator with thesig-nal amplitude reaching the amplitude detector to' introduce-'the'said first attenuation and to leave it in if the residual then reaching the amplitude detector is'above an arbitrary small value-andnext-to remove it if the residual-is less than this-small value; a circuit associated with the control circuit to transmit an off signal it the attenuator is left in and an o "--signal i f itis removed, each succeeding pulse in-coordination with the residual then reaching-the amplitude detector operating in turn in the same manner through the control circuit to introduce, and then'remove if necessary, its attenuator and to transmit corresponding signals, the 'n attenuators being graded in size fromthe srnallest oivalue Ao'to the largest of value 2 A0 the total attenuation-introduced and left in at-the end of a cycle being proportional on-a decibel basisto the s-ampleamplitude, thesizeof the steps increasing in-accorclance'with a binary counting system.

14. In a communication system, apparatus for sampling a signalling wave atregular occurring instants of time, a source of high frequency alternating current, means for maintaining the magnitude of said alternating current between said instants of timeat. a value'determined-by the magnitude of said signaling wave at the last instant of the sample, a source of reference voltage, apparatus for comparing a fraction of said alternating current with said reference voltage, equipment for sending either oneoi two-signaling conditions, and apparatus responsive to said comparison apparatus for controlling which of said two signaling conditions are transmitted incidenttto' said comparison.

15. In :a communication: system, apparatus for samplingta signaling wave-at regular-occurring instants;of time, a source ofhi'gh frequency aiternating current, means for maintaining the magnitude of said alternating current between said instants of time at a value determined by the magnitude of said'si-gnaling wave at the-last instant of the sample, a source of reference voltage, apparatus for comparing a fraction-of said alternating current with said reference voltage, equipmentfor sending either one of two signaling conditionshand apparatus responsive to said comparison. apparatus for controlling which-of saidtwo signaling conditions are transmitted incident to said comparison, and equipment for thereafter changing the fraction of i said alternating. current. and repeating the processof comparison-and signal transmission.

16. A. signal"transmission systemcomprising a sourceof electrical current, a source of reference voltage, switching-equipment for comparing a fraction of saidcurrent-withsa id reference voltage, andsignal transmitting equipment for transmitting a signal condition of a nature controlled by said comparison.

17. A signal transmission system' -com'prising a source of electrical current, a=s0urce or reference voltage, apparatus for comparing a fraction 

